Photonic quantum ring laser and fabrication method thereof

ABSTRACT

A photonic quantum ring (PQR) laser includes an active layer having a multi-quantum-well (MQW) structure and etched lateral face. The active layer is formed to be sandwitched between p-GaN and n-GaN layers epitaxially grown on a reflector disposed over a support substrate. A coating layer is formed over an outside of the lateral faces of the active layer, an upper electrode is electrically connected to an upper portion of the n-GaN layer, and a distributed Bragg reflector (DBR) is formed over the n-GaN layer and the upper electrode. Accordingly, the PQR laser is capable of oscillating a power-saving vertically dominant 3D multi-mode laser suitable for a low power display device, prevent the light speckle phenomenon, and generate focus-adjusted 3D soft light.

DESCRIPTION

1. Technical Field

The present invention relates in general to a semiconductor laser, andmore particularly, to a GaN-based photonic quantum ring laser suitablefor oscillating a multi-mode laser used in a low power display device,and its fabrication method.

2. Background Art

As well-known in the art, light emitting diodes (LEDs) are the focus ofattention in the display field because of their outstanding featuressuch as anti-vibration, high reliability, low power consumption, and soon.

Such LEDs have been used in a broad range of industrial applications ofdiverse fields, e.g., backlight sources of mobile displays, signposts onhighways, airport signs, stock quotation boards, subway guide boards,light emitters installed in vehicles and the like, and also recentlyapplied to traffic signal lamps to reduce energy consumption.

In particular, GaN-based LEDs can easily produce white light by applyingyellow phosphor powder to cause the combination of yellow and bluelight, the complementary colors. However, they are structurally limitedto enhance light extraction efficiency in that laser beams are generatedonly by spontaneous emission rather than stimulated emission.

Therefore, studies have been made about a resonant cavity LED (RCLED),which is configured by adding a resonator to an LED to improve thestraightness and intensity of light, or to reduce the full-width halfmaximum (FWHM) to several nanometers such that power consumption can bereduced without impairing the brightness. More improved resonantcavities of vertical cavity surface emitting laser (VCSEL) were alsoattempted for vertical lasing via stimulated emission.

However, the RCLED and the VCSEL have a drawback in that they require ahigh voltage or much current compared with their outputs, resulting in agreat heat loss to occur. In order to remedy the LED problem, there wereattempts of using a roughly patterned substrate (PS) LED or developingan LED with a roughened surface (RS), but neither of them could actuallysolve the high voltage problem.

To resolve the deficiency in the existing RCLED and VCSEL, manyresearches are actively in process to develop a 3D VCSEL type photonicquantum ring (PQR) laser operating with lower power consumption. The PQRlaser, unlike the VCSEL laser, does not require a high current and ishighly resistant to heat, whereby photo-conversion and thermalcharacteristics are remarkably improved, compared with any other LEDs.

DISCLOSURE OF INVENTION Technical Problem

It is, therefore, a primary object of the present invention to provide aphotonic quantum ring laser (PQR) suitable for a low power displaydevice, by forming a reflector over and underneath a GaN-basedmulti-quantum-well (MQW) structure to oscillate a power-savingvertically dominant 3D multi-mode laser, and its fabrication method.

Another object of the present invention is to provide a photonic quantumring laser capable of preventing the light speckle phenomenon andgenerating focus-adjusted 3D soft light, by forming a reflector over andunderneath a GaN-based MQW structure to oscillate a power-savingvertically dominant 3D multi-mode laser, and its fabrication method.

Technical Solution

In accordance with an aspect of the present invention, there is provideda photonic quantum ring (PQR) laser, including:

an active layer having a multi-quantum-well (MQW) structure and etchedlateral face;

n-GaN and p-GaN layers, which are epitaxially grown, with the activelayer sandwiched therebetween;

a reflector disposed over a support substrate and having the p-GaN layerthereon;

a coating layer formed over an outside of lateral faces of the activelayer;

an upper electrode electrically connected to an upper portion of then-GaN layer; and

a distributed Bragg reflector (DBR) formed over the n-GaN layer and theupper electrode.

In accordance with another aspect of the present invention, there isprovided a fabrication method of a photonic quantum ring laser,including the steps of:

forming, over a sapphire substrate, an n-GaN layer, an active layer, ap-GaN layer, and a reflector in order;

adhering a support substrate over the reflector and removing thesapphire substrate;

performing a selective etching to expose a lateral face of the activelayer and forming a coating layer over an outside of the lateral facesof the active layer;

forming, over the coating layer, an upper electrode electricallyconnected to the n-GaN layer; and

forming a distributed Bragg reflector (DBR) in a region having the samewidth as the active layer and the n-GaN layer over the upper electrodeand the n-GaN layer.

ADVANTAGEOUS EFFECTS

In accordance with the present invention, the photonic quantum ringlaser is fabricated by depositing a DBR and a metal reflector over andunderneath a GaN-based active layer so that it can oscillate apower-saving vertically dominant 3D multi-mode laser suitable for a lowpower display device, prevent the light speckle phenomenon, and generatefocus-adjusted 3D soft light.

In addition, the use of such a PQR laser increases a response speed upto the GHz range compared with that of conventional LEDs. This leads tosignificant improvement in performance such as high frequency modulationand pulse modulation such that the application range of LEDs can beexpanded not only to lighting devices but also to cellular phones,next-generation display devices, and so on.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodiments,given in conjunction with the accompanying drawings, in which:

FIG. 1 shows a structural diagram of a 3D toroidal cavity of a photonicquantum ring (PQR) laser in accordance with an embodiment of the presentinvention;

FIG. 2 illustrates a cross-sectional view showing the structure of ablue PQR laser in accordance with an embodiment of the presentinvention;

FIG. 3 graphically shows a luminous state of a blue PQR laser inaccordance with the present invention;

FIGS. 4 to 11 illustrate a fabricating process of a blue PQR laser inaccordance with an embodiment of the present invention;

FIG. 12 shows an oscillation spectrum in relation to injection currentin an array chip of PQR lasers in accordance with the present invention;and

FIG. 13 shows an L-I curve of a PQR laser in accordance with the presentinvention.

Best Mode for Carrying Out the Invention

As will be discussed below, the technical gist of the present inventionis to fabricate a blue photonic quantum ring (PQR) laser by forming,over a sapphire substrate, an n-GaN layer, an active layer, a p-GaNlayer, and a reflector in sequence; adhering a support substratethereover; removing the sapphire substrate; patterning to expose theactive layer; forming a passivation layer; coating polyimide on theoutside thereof and planarizing the same; and forming an upper electrodeand a distributed Bragg reflector (DBR) thereover. Thus, the use of suchtechnical schemes makes it possible to overcome the prior art problems.

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings.

FIGS. 1A and 1B show the structure of a 3D toroidal cavity of a PQRlaser in accordance with an embodiment of the present invention. Inparticular, FIG. 1A illustrates a perspective view of a portion of thePQR laser and FIG. 1B illustrates a pattern of the helical wave with aBessel field profile generated in the PRQ laser.

The PQR laser shown in FIG. 1A generally forms a toroidal cavity typewhispering gallery (WG) mode under 3D Rayleigh-Fabry-Perot (RFP)conditions by way of vertically confining photons with DBRs arrangedover and underneath an active multi-quantum-well (MQW) layer andhorizontally confining photons with total reflection occurring around asidewall including the active MQW layer of the PQR laser. At this time,carriers on the active QW planes of the MQW layer within a ring definedas a toroid are re-distributed in the form of concentric circles ofquantum wires (QWRs) by a photonic quantum corral effect (PQCE), suchthat photons are produced by electron-hole recombination.

Moreover, the oscillation mode wavelength and the inter-mode spacing(IMS) of the PQR laser can be adjusted by reducing the radius R of thePQR laser. Specifically, by reducing the radius R of the PQR laser, itis also possible to adjust the IMS of the PQR laser that oscillatesdiscretely at multi-wavelengths within an envelope wavelength range forthe PQR laser of several nm to several tens of nm. The IMS adjustment inturn makes it possible to determine the number of oscillation modeswithin the entire envelope of the PQR laser, thereby controlling theamount of power consumption in the PQR laser.

In such a PQR laser, a Rayleigh ring is defined along thecircumferential edge of the active MQW layer in the 3D toroidal RFPcavity. The PQR laser is driven at an ultra-low threshold current statewhile inducing electron-hole recombination of certain QWR concentriccircles in the Rayleigh ring. Consequently, the PQR laser oscillating atthe ring surpasses the light-emitting performance of self-transitiontype LEDs in its center, and its output wavelength can be maintainedstably by virtue of QWR characteristics.

FIG. 2 is a cross-sectional view showing the structure of a blue PQRlaser in accordance with an embodiment of the present invention.

Referring to FIG. 2, the blue PQR laser includes a semiconductorsubstrate 200, a reflector 202, a p-GaN layer 204, an active layer 206,an n-GaN layer 208, a passivation layer 210, a coating layer 212, anupper electrode 214, and a DBR 216. The semiconductor substrate 200serves to support its upper structures formed thereon and may be madeof, e.g., silicon (Si) or other metals.

The active layer 206, the n-GaN layer 208, and the DBR 216 are formed ina region of the same width over the semiconductor substrate 200, and thereflector 202 is disposed on the semiconductor substrate 200 and is madeof a metal such as silver (Ag) or the like to have reflectivity as highas 90% or more.

Moreover, the active layer 206 is sandwiched between the p-GaN layer 204and the n-GaN layer 208 which are epitaxially grown, over the reflector202, using magnesium (Mg) and silicon (Si), respectively. The activelayer 206 has an MQW structure, in which a GaN layer as a barrier layerand an InGaN or AlInGaN layer as a well layer are alternately depositedto form, e.g., a four layer laminate. According to this MQW structure,for example, four quantum wells may be formed, and blue, green, and redlights may be generated by adjusting the material composition ratio(e.g., indium (In), aluminum (Al), etc.), followed by selecting a lightemitting wavelength range. Needless to say, it is possible to generatewhite light by coating the blue PQR laser with yttrium aluminum garnet(YAG).

Meanwhile, the reflector 202, the p-GaN layer 204, the active layer 206,and the n-GaN layer 208 are selectively etched in the form of a circularmesa (i.e., etched depending on a specific photoresist pattern) by dryetching, e.g., chemically assisted ion beam etching (CAIBE). In order toprotect the circular mesa, the passivation layer 210 made of siliconnitride (e.g., SiN_(x), etc.) or silicon oxide (e.g., SiO₂, etc.) isprovided, and polyimide is coated and then planarized over the outsidethereof to form the coating layer 212, through which a toroidal typecavity is formed. At this point, the passivation layer 210 has beenremoved through a planarization process for example, so as to expose anupper surface of the n-GaN layer 208 in contact with the upper electrode214 and the DBR 216.

After that, the upper electrode 214, which is an n-type ohmic electrodemade of Ti/Al for example, is formed in contact with the N-GaN layer208, and then the DBR 216 is disposed thereover. Specifically, the DBR216 is formed by alternately laminating layers of a dielectric materialsuch as TiO₂/SiO₂, SiN_(x)/SiO₂, or the like, at least one or more timeso that it has reflectivity as high as 60% or more. Of course, the DBR216 may be formed in a GaN/Al_(x)Ga_(1−x)N structure.

As an example, FIG. 3 graphically depicts a luminous state of a blue PQRlaser in accordance with the present invention. In particular, FIG. 3provides the photograph of a CCD image of an array of blue PQR laserswhere a PQR laser is about 20 μm in diameter and PQR lasers are spacedapart from one another by about 15 μm. This photograph of the CCD imagewas acquired by filtering or blocking, by a neutral density filter(NDF), most of the light emission amount that causes the saturation ofCCD (pixels) as the current injection increases. As known from the CCDimage, the inventors could observe a strongly oscillated state of thePQR laser, and learned that the light speckle phenomenon was preventedby the PQR laser and instead the focus-adjusted 3D soft light wasgenerated. Needless to say, the size and array density of the PQR lasermay be varied, if needed.

Now, a fabricating process of the blue PQR laser having the structure asset forth above will be explained in detail with reference to FIGS. 4 toFIG. 11.

FIGS. 4 to FIG. 11 illustrate a fabricating process of a blue PQR laserin accordance with the present invention.

Referring to FIG. 4, an n-GaN layer 402, an active layer 404, and ap-GaN layer 406 are formed sequentially on a sapphire substrate 400. Inthis structure, the sapphire substrate 400 may be replaced with asubstrate made of silicon carbide (SiC), silicon (Si), or GaAs. Then-GaN layer 402 is grown at a temperature in a range of 1000-1200° C.,and then doped with silicon as a dopant. For the growth of the n-GaNlayer 402, a low-temperature GaN buffer layer may be formed at arelatively low temperature in a range of 500-600° C., thereby preventingthe occurrence of crack due to a difference in lattice parametersbetween the n-GaN layer 402 and the sapphire substrate 400.

The active layer 404 is formed to have an MQW structure in which a welllayer and a barrier layer are alternately formed. That is, an InGaN orAlInGaN layer as a well layer and a GaN layer as a barrier layer arealternately deposited to form a four layer laminate for example.According to this MQW structure, for example, four quantum wells may beformed, and one of blue, green, and red light may be generated byadjusting the composition ratio of materials (e.g., indium (In),aluminum (Al), etc.), followed by selecting a light emitting wavelengthrange. Needless to say, it is possible to generate white light bycoating the blue PQR laser with yttrium aluminum garnet (YAG).Thereafter, the p-GaN layer 406 doped with magnesium (Mg) is grown overthe active layer 404 under gas atmosphere such as hydrogen (H) or thelike.

Next, as shown in FIG. 5, a reflector 408 made of silver (Ag) or thelike is formed over the sapphire substrate 400 on which the n-GaN layer402, the active layer 404, and the p-GaN layer 406 are provided in orderas mentioned above. In particular, the reflector 408 is prepared in amanner that its reflectivity is more than 90%.

Then, referring to FIG. 6, a semiconductor substrate 410 is adhered overthe reflector 408 in order to support the structure as noted above, andthe sapphire substrate 400 is then removed by a laser lift-off (LLO)method by using an excimer laser. Next, the surface of the n-GaN layer402, from which the sapphire substrate 400 is removed, undergoes acleaning process.

Subsequently, as shown in FIG. 7, the n-GaN layer 402, the active layer404 and the p-GaN layer 406 are subjected to a selective etching to acertain thickness of the p-GaN layer 406 by dry etching such as CAIBE(that is, etched depending on a specific photoresist pattern), so that acircular mesa with a smooth lateral face is formed. In result of the dryetching, for example, an 8x8 array of the PQR lasers may be arranged inan area of a standard LED of about 300-400 μm in size. While thefabrication method in accordance with the embodiment of the presentinvention has been described as performing the dry etching down to acertain thickness of the p-GaN layer 406, it may also acceptable tocarry out the dry etching down to the p-GaN layer 406 or to thereflector 408 as depicted in FIG. 2, such that the layers from the n-GaNlayer 402 to the active layer 404 are fully exposed. In other words, thedry etching is done selectively to completely expose only the lateralface of the active layer 404.

Next, referring to FIG. 8, a passivation layer 412 made of SiN_(X),SiO₂, or the like is deposited on the entire upper surface on which thecircular mesa is formed, and is then selectively removed byplanarization to expose the upper face of the n-GaN layer 402 in contactwith an upper electrode 416 and a DBR 418 to be formed later. Then,polyimide is coated on the passivation layer 412, and undergoes apolyimide (PI) planarization process for exposing the n-GaN layer 402,such that a coating layer 414 is formed in a toroidal cavity structure,as shown in FIG. 9.

In succession, a metal material such as Ti/Al is deposited on thecoating layer 414 and then dry etched to selectively form an upperelectrode 416 as an n-type ohmic electrode as shown in FIG. 10, so as tobe in contact with the n-GaN layer 402. Alternatively, the upperelectrode 416 may be formed through an ohmic contact which is made byetching for the formation of an n-type ohmic electrode and then byheat-treating it under predetermined temperature and time conditions innitrogen or nitrogen-mixed gas atmosphere.

Next, a DBR 418 is formed over the upper electrode 416 by using adielectric material such as TiO₂/SiO₂, SiN_(x)/SiO₂ or the like, and aselective etching is performed such that the DBR 418 has the same widthas that of the active layer 404 and the n-GaN layer 402 as shown in FIG.11, thus completing the fabrication process of the PQR laser. Here, theDBR 418 is obtained by alternately laminating layers of a dielectricmaterial such as TiO₂/SiO₂, SiN_(x)/SiO₂ or the like, at least one timeso that it has reflectivity as high as 60% or more. As an example, ifthe DBR 418 has a two-alternate-layer laminate structure, it may be madeof TiO₂/SiO₂/TiO₂/SiO₂, SiN_(x)/SiO₂/SiN_(x)/SiO₂, etc. Of course, theDBR 418 may be formed in a GaN/Al_(x)Ga_(1−x)N structure.

To be short, the blue PQR laser can be fabricated by forming, over asapphire substrate, an n-GaN layer, an active layer, a p-GaN layer, anda reflector in sequence; adhering a support substrate over thereflector; removing the sapphire substrate; performing a selectiveetching until the active layer is exposed; forming a passivation layer;coating polyimide on the outside thereof and planarizing the polyimidecoating; and forming an upper electrode and a distributed Braggreflector thereover.

FIG. 12 shows an oscillation spectrum in relation to injection currentin an array chip of PQR lasers in accordance with the present invention.Particularly, FIG. 12 shows an oscillation spectrum in different levelsof current injected into an 8x8 array chip of the PQR lasers, eachhaving a diameter of about 20 μm. It exhibits the entire envelope ofabout 40 nm FWHM, which is very similar to the spectrum profile of atypical GaN LED and shows a multi-mode lasing spectrum having a FWHM_(m)of 3-4 nm oscillating in that range.

FIG. 13 presents an L-I curve of, e.g., a 2x8 array of PQR lasers inaccordance with the present invention, in which a threshold current ofthe 2x8 array of PQR lasers is approximately 200 μA, with about 13 μAfor each PQR laser.

While the invention has been shown and described with respect to thepreferred embodiments, it will be understood by those skilled in the artthat various changes and modification may be made without departing fromthe scope of the invention as defined in the following claims.

1. A photonic quantum ring (PQR) laser, comprising: an active layerhaving a multi-quantum-well (MQW) structure and etched lateral face;p-GaN and n-GaN layers, which are epitaxially grown, with the activelayer sandwiched therebetween; a reflector disposed over a supportsubstrate and having the p-GaN layer thereon; a coating layer formedover an outside of lateral faces of the active layer; an upper electrodeelectrically connected to an upper portion of the n-GaN layer; and adistributed Bragg reflector (DBR) formed over the n-GaN layer and theupper electrode.
 2. The PQR laser of claim 1, wherein the reflector isformed by using silver (Ag).
 3. The PQR laser of claim 1, wherein theDBR is formed by using a dielectric material in a TiO₂/SiO₂ orSiN_(x)/SiO₂ structure.
 4. The PQR laser of claim 1, wherein the DBR isformed in a GaN/Al_(x)Ga_(1−x)N structure.
 5. The PQR laser of claim 1,wherein the active layer is formed by using InGaN or InAlGaN.
 6. The PQRlaser of claim 5, wherein the PQR laser selects a light-emittingwavelength range by adjusting a composition ratio of indium (In)component and aluminum (Al) component contained in the active layer. 7.The PQR laser of claim 1, wherein the PQR laser determines an inter-modespacing (IMS) by adjusting the radius thereof.
 8. The PQR laser of claim1, further comprising: a passivation layer formed between the activelayer and the coating layer.
 9. The PQR laser of claim 8, wherein thepassivation layer is formed by using SiN_(x) or SiO₂.
 10. A fabricationmethod of a photonic quantum ring laser, comprising the steps of:forming, over a sapphire substrate, an n-GaN layer, an active layer, ap-GaN layer, and a reflector in order; adhering a support substrate overthe reflector and removing the sapphire substrate; performing aselective etching to expose a lateral face of the active layer andforming a coating layer over an outside of the lateral faces of theactive layer; forming, over the coating layer, an upper electrodeelectrically connected to the n-GaN layer; and forming a distributedBragg reflector (DBR) in a region having the same width as the activelayer and the n-GaN layer over the upper electrode and the n-GaN layer.11. The method of claim 10, further comprising the step of: beforeforming the coating layer, forming a passivation layer on the lateralface of the active layer exposed by the selective etching.
 12. Themethod of claim 10, wherein the passivation layer is formed by usingSiN_(X) or SiO₂.
 13. The method of claim 10, wherein the reflector isformed by using silver (Ag).
 14. The method of claim 10, wherein the DBRis formed by using a dielectric material in a TiO₂/SiO₂ or SiN_(x)/SiO₂structure.
 15. The method of claim 13, wherein the DBR is formed in aGaN/Al_(x)Ga_(1−x)N structure.
 16. The method of claim 10, wherein theactive layer is formed by using InGaN or InAlGaN.
 17. The method ofclaim 10, wherein the sapphire substrate is removed by a laser lift-off(LLO).
 18. The method of claim 10, wherein the coating layer is formedby performing polyimide coating and polyimide planarization.